OIC thanks, I did a specifically Surefire search.
Cheers
Update:
Easiest way is to buy a lens cover FM63 new, used, or without a lens. The cheapest is to go on eBay and buy one without a lens because you replace it with a Lexan lens anyway.
Just like this:
Now use the forum post link to cut out a Lexan lens. Either replace the lens in the filter cap or put the lens in front of the light and slide the filter cap over both. I used a rubber spacer to keep the Lexan lens away from the front of the surefire lens. This way I can still put an IR filter lens in the filter flip cap.